Seal materials for vessel caps that contain polyvinyl chloride (PVC) have long been used in the packaging industry.
The use of PVC-containing compositions in packaging materials is generally no longer desirable however for many reasons. When household waste is incinerated, acidic gases are produced from halogen plastics and the escape of said gases into the atmosphere is harmful. In addition, even small amounts of PVC interfere with the material recycling of plastic waste. Furthermore, PVC-based seal elements require the use of plasticizers, which may potentially migrate into the food contained in the vessel and are therefore questionable for health reasons.
The object of the present invention is to provide a PVC-free sealing compound (also referred to hereinafter as a polymer compound) for vessel caps, in particular for the packaging of foods. Foods (including beverages such as juices and the like) are often packaged in vessels made of glass or plastic, which in many cases then have a screw lid. The term “screw lid” is representative here for vessel caps that, in the filled and closed state, are engaged with the vessel by means of a thread. To open the vessel, the cap has to be rotated relative to the vessel, wherein the seal of the cap lifts from the vessel edge and the vacuum (often) provided in the vessel is cancelled. The cap can be separated from the vessel by such a rotation. The known PVC-containing seal materials have the processing and performance characteristics necessary for this purpose. It should thus be noted that a PVC-free sealing compound is only then a seal material of commercial interest for vessel caps if the PVC-free sealing compound has quite specific physical-chemical properties not inferior to the main properties of PVC-containing seal materials.
Since many foods and beverages are sterilized once the vessel has been filled and closed, a seal insert that withstands such measures is particularly desirable.
It has now surprisingly been found that this suitability for sterilization measures can be established on the seal material in a simple manner by means of dynamic mechanical thermal analysis (DMTA).
DMTA is a known method: appropriate measuring devices are commercially available. The principle of DMTA can be simply described: A sinusoidal oscillating force is applied to a material sample. The deformation of the material is measured. Here, both the amplitude and the phase shift of the deformation with respect to the applied force are determined. The viscoelastic properties of a sample can be determined from the measured values as a function of time and temperature. Besides the glass transition temperature Tg, these include the storage module G′ and the loss module G″ of the material. With most devices, forced oscillations outside the resonance are used. The sample is mechanically subjected to bending load, strain, or shear load with defined frequency and at defined temperature. The mechanical loading is applied here either separately after static medium load and dynamic component, or in one step by means of deformers. The dynamic loading is generally produced by an electrodynamic oscillator, which, depending on the device, covers a specific frequency range. Temperature control is possible with most devices in a range from −100° C. to over +300° C. During the measurement process, the force and deformation signals are recorded, and the phase angle between the two signals is determined by means of Fourier analysis.
Alternatively, measurements of this type can also be carried out in an oscillation rheometer on polymer melts. To this end, a circular disc-shaped sample of defined layer thickness is introduced into a cone and plate system and is heated by 5K min−1 to 180° C. and is measured in a cooling-heating cycle.
Sterilizable seal materials can be identified in accordance with the invention in that they basically demonstrate the behavior defined in claim 1.
In any individual case, attention should be paid to the conditions of the intended sterilization treatment when assessing the DMTA data. The maximum possible sterilization temperature is lower, the higher is the pressure on the cap. Generally, the inflection point (without counterpressure) should be at least 10° C. higher than the desired sterilization temperature. With counterpressure, an even greater distancing of 20° C. and more may be necessary.
The heating curve for the phase angle tan (delta) may demonstrate more than one inflection point, for example if individual components already exhibit a phase transition (which then occurs at relatively low temperatures) before the material softens as a whole.
In such cases, such inflection points should not be considered, but merely the inflection point that corresponds to the softening of the seal material as a whole.
More specific requirements for example include the following aspects:                The material composition is to be selected such that undesirable substances are avoided. The sealing compound should therefore not contain substances that are classified as presenting a health risk, in particular plasticizers, such as phthalates; semicarbazide and sources thereof, in particular ADC and OBSH; 2-ethylhexanoic acid and sources thereof; organic tin compounds, primary aromatic amines, bisphenols, nonylphenol; BADGE; photoinitiators; perhalogenated compounds; melamine.                    For some applications, the presence of larger contents of liquid substances (that is to say substances that are liquid at RT) is undesirable. Then, the content of such substances (such as white oil) should be at most 10%, preferably less restricted, and in some cases the sealing compound should have no traceable contents of such liquid substances.            If bisphenol-A and melamine are to be avoided, coatings that do not contain such substances are used for the vessel cap. The seal material should then be composed such that a lasting adhesive bond is achieved with such coatings.            The material composition should be selected such that the seal material satisfies even challenging requirements during use.            The seal material should thus preferably be usable under pasteurization or even sterilization conditions, that is to say should withstand a pasteurization (up to 98° C.) or a sterilization (generally above 100° C., often above 105° C. or above 110° C., or even above 120°, up to 132° C.).            For some uses, the seal material should have a barrier function, that is to say should reduce or prevent the infiltration of undesirable substances into the vessel.            For specific applications, it should be possible to provide the seal material with absorbing additives (for example oxygen absorbers) or scavenger substances.                        The seal material must have the required processing characteristics.                    In principle, it must soften thermally to a sufficient extent so as to be useable on conventional processing machines (in particular in injection molding methods, but also for extrusion with subsequent stamping or compression molding).            It must therefore have the necessary sealing properties after introduction into the vessel cap and cooling to the desired application temperature (generally RT, but possibly also at lower temperatures, for example in a chilling cabinet).            It must also be possible to introduce the seal material over the entire area for small vessel caps.            For PT caps (Press-on Twist-off® caps), the seal material must form both the seal and the inner thread of the cap, and it must therefore be possible to apply the seal material (as what is known as a “contoured ring”) both to the inner face and to the skirt of the cap, and the seal material must also be able to form the thread elements when the cap is pressed on.            For some applications, the seal material should be able to form the seal insert “out shell”, that is to say outside the cap, the seal insert then being inserted as a finished ring seal or the like into the vessel cap.            The seal material is to be suitable in particular for metal caps and metal-plastic composites that may be coated on the inner face, however it is also to be suitable for plastic caps.                        The seal material must be suitable for conventional food packagings.                    The seal insert must be suitable for pasteurization (up to 98° C. or more) and should preferably also be suitable for sterilization (up to 132° C.).            The seal insert must withstand a post-treatment (pasteurization and the like) at counterpressure and evacuation; if necessary, it must have vacuum retention and barrier properties where applicable.            The seal insert is to be suitable for conventional vessels made of metal, plastic, glass, etc.            In the event of contact with the filled content, in particular fat-containing foods, alcoholic beverages and other products of lipophilic nature (compared to water), the seal insert is to deliver no components or minimal components to the filled content.            The seal insert must demonstrate sufficiently low twist-off values in order to be able to remove the vessel cap (possibly with cooling) with moderate force. At the same time, the seal must demonstrate its sealing effect over the intended lifetime (minimum shelf life) of the food.                        
The objects addressed by the present invention are achieved by the PVC-free compositions defined in the independent claims. Advantageous embodiments are defined in dependent claims.